Export the block size capability to guests.

• Use #defines for capability bits
• Export the VTBLK_F_BLK_SIZE capability
• Fix bug in calculating capacity: it is in 512-byte units, not the underlying sector size

This allows virtio-blk to have backing devices
with non 512-byte sector sizes e.g. /dev/cd0, and
4K-block harddrives.

Reviewed by: neel
MFC after: 3 days